The present invention relates to data processing systems and, more particularly, to a data processing system of the type having a memory system with a back-up or alternate memory.
One of the most troublesome problems facing the computer industry is the occurrence of faults or defects in integrated circuit chips. Such defects substantially affect the reliability and cost of computer systems, since modern computer systems are largely constructed from integrated circuits.
In the past, one approach to overcoming the problems posed by defective integrated circuit chips has been to maintain strict quality control measures in order to detect those chips that have faults or defects. The defective chips can be separated from the non-defective chips and discarded. The price that one pays for each of the remaining good chips that is free of defects usually depends on the yield obtained by the chip manufacturer. If the yield is good, with very few chips having defects, the price per chip may be low. However, if the yield is poor, with most of the manufactured chips having defects, the price for one of the few good chips will be relatively high.
Particular problems are faced in the case of integrated circuits that are used to construct a memory system, since defects can occur at storage or memory cells and can prevent data from being reliably stored at such defective cells. A bit error would normally occur in the data stored at each defective cell.
It has been proposed in the past to use memory devices or chips having defective cells in a primary memory by providing a back-up or alternate memory. The alternate memory stores data that would otherwise be stored at the defective cell locations in the memory chips of the primary memory. Circuitry is provided for accessing data in the alternate memory so that when data is to be read from the defective cells in the primary memory, the data in the alternate memory is substituted in its place.
One drawback to systems employing alternate memories is the additional cost associated with the alternate memory. The need for additional memory or memory chips for the alternate memory increases the cost of the memory system and it may not be any less expensive than using only higher-priced good memory chips. In particular, since a bit addressable alternate memory requires a large number of address lines and would need to be accessed several times when a word having multiple bit errors is accessed in the primary memory, it has been found necessary to construct the alternate memory so that it will store an entire data word or byte to replace each data word or byte in the primary memory that has one or more bit errors. As a result, in a primary memory having a fairly significant number of cell defects, the size and cost of the alternate memory may need to be large.